If a Cohen wishes to marry an orthodox jewess who is a divorcee, my understanding is that this is impossible. However, if the Cohen is able to cast doubt upon whether he is in fact a Cohen or not, it might become evident that he MAY not in fact be a Cohen after all. The question is, if he does cast doubt that he may not be a Cohen, would this prevent his fathers, uncles and cousins from continuing their duties as Cohanim?

If we can prove beyond any doubt that he is not a Cohen, all similar status family members will be disqualified as well, If however the proof is uncertain, it will not necessarily affect the others, This is a question for the Beth Din in charge to decide how strong it the proof and its implications.